Müller cells as a target for retinal therapy

Visual stimulation and brain-derived neurotrophic factor (BDNF) have protective effects in experimental autoimmune uveoretinitis

AIMS

To investigate the therapeutic potential of visual stimulation (VS) and BDNF in murine experimental autoimmune uveoretinitis (EAU).

MAIN METHODS

Mice were immunized by subcutaneous injection of interphotoreceptor retinoid-binding protein in Freund's complete adjuvant and intravenous injection of pertussis toxin, and were then exposed to high-contrast VS 12 h/day (days 1-14 post-immunization). EAU severity was assessed by examining clinical score, visual acuity, inflammatory markers, and immune cells in the retina. The transcriptome of activated retinal cells was determined by RNA-seq using RNA immunoprecipitated in complex with phosphorylated ribosomal protein S6. The retinal levels of protein products of relevant upregulated genes were quantified. The effect of BDNF on EAU was tested in unstimulated mice by its daily topical ocular administration (days 8-14 post-immunization).

KEY FINDINGS

VS attenuated EAU development and decreased the expression of pro-inflammatory cytokines/chemokines and numbers of immune cells in the retina (n = 10-20 eyes/group for each analysis). In activated retinal cells of control mice (n = 30 eyes/group), VS upregulated genes encoding immunomodulatory neuropeptides, of which BDNF and vasoactive intestinal peptide (VIP) also showed increased mRNA and protein levels in the retina of VS-treated EAU mice (n = 6-10 eyes/group for each analysis). In unstimulated EAU mice, BDNF treatment mimicked the protective effects of VS by modulating the inflammatory and stem cell properties of Müller cells (n = 5 eyes/group for each analysis).

SIGNIFICANCE

VS effectively suppresses EAU, at least through enhancing retinal levels of anti-inflammatory and neuroprotective factors, VIP and BDNF. Our findings also suggest BDNF as a promising therapeutic agent for uveitis treatment.

Role of inflammation in a rat model of radiation retinopathy

Radiation retinopathy (RR) is a major side effect of ocular tumor treatment by plaque brachytherapy or proton beam therapy. RR manifests as delayed and progressive microvasculopathy, ischemia and macular edema, ultimately leading to vision loss, neovascular glaucoma, and, in extreme cases, secondary enucleation. Intravitreal anti-VEGF agents, steroids and laser photocoagulation have limited effects on RR. The role of retinal inflammation and its contribution to the microvascular damage occurring in RR remain incompletely understood. To explore cellular and vascular events after irradiation, we analyzed their time course at 1 week, 1 month and 6 months after rat eyes received 45 Gy X-beam photons. Müller glial cells, astrocytes and microglia were rapidly activated, and these markers of retinal inflammation persisted for 6 months after irradiation. This was accompanied by early cell death in the outer retina, which persisted at later time points, leading to retinal thinning. A delayed loss of small retinal capillaries and retinal hypoxia were observed after 6 months, indicating inner blood‒retinal barrier (BRB) alteration but without cell death in the inner retina. Moreover, activated microglial cells invaded the entire retina and surrounded retinal vessels, suggesting the role of inflammation in vascular alteration and in retinal cell death. Radiation also triggered early and persistent invasion of the retinal pigment epithelium by microglia and macrophages, contributing to outer BRB disruption. This study highlights the role of progressive and long-lasting inflammatory mechanisms in RR development and demonstrates the relevance of this rat model to investigate human pathology.

CRX haploinsufficiency compromises photoreceptor precursor translocation and differentiation in human retinal organoids

BACKGROUND

The CRX-associated autosomal dominant retinopathies suggest a possible pathogenic mechanism of gene haploinsufficiency. However, based on reported human patient cases and studies with mouse models, it is hard to confirm the specific weight of haploinsufficiency in pathogenesis due to the interspecies gaps between gene expression and function.

METHODS

We created monoallelic CRX by replacing one allele with tdTomato in human embryonic stem cells (hESCs) and subsequently dissect pathogenesis in hESCs-derived retinal organoids. We used transcriptome and immunofluorescence analyses to dissect phenotypic differences between CRX-monoallelic knockout and control wildtype organoids. For location analysis of CRX+ cells, a CRX-expression-tracing system was constructed in control hESCs. We implemented long-term live-cell imaging to describe the translocation of CRX+ cells between two groups in early organoid differentiation. The expression pattern of these dynamic differences was validated using RNA-seq and immunofluorescence assays.

RESULTS

We identified delayed differentiation of outer nuclear layer (ONL) stratification along with thinner ONL, serious loss of photoreceptor outer segments, as well as downregulated expression of gene for phototransduction and inner/outer segment formation. By live-cell imaging and immunostaining, we observed the overtension of actomyosin network and the arrested translocation of monoallelic CRX+ cells in the early stage of retinal differentiation.

CONCLUSIONS

We confirmed that gene haploinsufficiency is the mechanism for the dominant pathogenicity of CRX and discovered that CRX regulated postmitotic photoreceptor precursor translocation in addition to its specification of photoreceptor cell fates during human retinal development. These findings revealed a new underlying mechanism of CRX dominant pathogenesis and provided a new clue for the treatment of CRX-associated human retinopathies.

Adeno-associated virus vectors for retinal gene therapy in basic research and clinical studies

Retinal degenerative diseases, including glaucoma, age-related macular degeneration, diabetic retinopathy, and a broad range of inherited retinal diseases, are leading causes of irreversible vision loss and blindness. Gene therapy is a promising and fast-growing strategy to treat both monogenic and multifactorial retinal disorders. Vectors for gene delivery are crucial for efficient and specific transfer of therapeutic gene(s) into target cells. AAV vectors are ideal for retinal gene therapy due to their inherent advantages in safety, gene expression stability, and amenability for directional engineering. The eye is a highly compartmentalized organ composed of multiple disease-related cell types. To determine a suitable AAV vector for a specific cell type, the route of administration and choice of AAV variant must be considered together. Here, we provide a brief overview of AAV vectors for gene transfer into important ocular cell types, including retinal pigment epithelium cells, photoreceptors, retinal ganglion cells, Müller glial cells, ciliary epithelial cells, trabecular meshwork cells, vascular endothelial cells, and pericytes, via distinct injection methods. By listing suitable AAV vectors in basic research and (pre)clinical studies, we aim to highlight the progress and unmet needs of AAV vectors in retinal gene therapy.

Effects of Apelin on the fibrosis of retinal tissues and Müller cells in diabetes retinopathy through the JAK2/STAT3 signalling pathway

Retinal fibrosis was a key characteristic of diabetes retinopathy (DR). Apelin was found to be a candidate for tissue fibrosis. Nevertheless, the role of Apelin in the Müller cells in DR remains unclear. This study identified the function and mechanism of Apelin in Müller cells and the fibrosis of retinal tissue. Western blot was carried out to detect the Apelin, GFAP, Collagen I, α-SMA, JAK2 and STAT3 protein levels. Masson staining was performed to display the histopathological changes in retinal tissue of diabetic mellitus (DM) rats. The immunofluorescence staining was conducted to evaluate the Apelin levels in the retinal tissue. The levels of GFAP, Collagen I and α-SMA in the retinal tissue of DM rats was visualised by the immunohistochemistry staining. The results showed that Apelin, GFAP, Collagen I andα-SMA expression was prominently elevated in the retinal tissue of DM rats and high glucose (HG)-exposed Müller cells. The results of Masson staining showed that the epiretinal fibrotic membrane was observed in DM rats. Apelin knockdown declined the GFAP, Collagen I andα-SMA levels. Besides, the protein levels of p-JAK2 and p-STAT3 were elevated in the HG-treated Müller cells, while Apelin knockdown declined them. FLLL32 treatment neutralised the role of Apelin. In conclusion, Apelin facilitated the fibrogenic activity of Müller cells through activating the JAK2/STAT3 signalling pathway, and thus inducing the retinal fibrosis in DR.

Induced pluripotent stem cell-derived extracellular vesicles overexpressing SFPQ protect retinal Müller cells against hypoxia-induced injury

Splicing factor proline/glutamine-rich (SFPQ) is expressed in induced pluripotent stem cells (iPSCs), which are reported to orchestrate hypoxic injury responses and release extracellular vesicles (EVs). Therefore, this study sought to explore the role of iPSC-derived EVs carrying SFPQ in hypoxia-induced injury to retinal Müller cells. We induced oxygen-glucose deprivation/reoxygenation (OGD/R) in Müller cells. SFPQ was overexpressed or knocked down in iPSCs, from which EVs were extracted. Müller cells were co-cultured with EVs, and the results indicated that SFPQ protein was transferred into retinal Müller cells by iPSC-derived EVs. We identified an interaction of SFPQ with HDAC1 in retinal Müller cells. Specifically, SFPQ recruited HDAC1 to downregulate HIF-2α by regulating its acetylation. The in vitro studies suggested that iPSC-derived EVs, SFPQ or HDAC1 overexpression, or HIF-2α silencing diminished cell injury and apoptosis but elevated proliferation in retinal Müller cells. The in vivo studies indicated that iPSC-derived EVs containing SFPQ curtailed apoptosis of retinal Müller cells, thus alleviating retinal ischemia/reperfusion (I/R) injury of rat model. Taken together, iPSC-derived EVs containing SFPQ upregulated HDAC1 to attenuate OGD/R-induced Müller cell injury via downregulation of HIF-2α.

Exploring the molecular makeup of support cells in insect camera eyes

Animals typically have either compound eyes, or camera-type eyes, both of which have evolved repeatedly in the animal kingdom. Both eye types include two important kinds of cells: photoreceptor cells, which can be excited by light, and non-neuronal support cells (SupCs), which provide essential support to photoreceptors. At the molecular level deeply conserved genes that relate to the differentiation of photoreceptor cells have fueled a discussion on whether or not a shared evolutionary origin might be considered for this cell type. In contrast, only a handful of studies, primarily on the compound eyes of Drosophila melanogaster, have demonstrated molecular similarities in SupCs. D. melanogaster SupCs (Semper cells and primary pigment cells) are specialized eye glia that share several molecular similarities with certain vertebrate eye glia, including Müller glia. This led us to question if there could be conserved molecular signatures of SupCs, even in functionally different eyes such as the image-forming larval camera eyes of the sunburst diving beetle Thermonectus marmoratus. To investigate this possibility, we used an in-depth comparative whole-tissue transcriptomics approach. Specifically, we dissected the larval principal camera eyes into SupC- and retina-containing regions and generated the respective transcriptomes. Our analysis revealed several common features of SupCs including enrichment of genes that are important for glial function (e.g. gap junction proteins such as innexin 3), glycogen production (glycogenin), and energy metabolism (glutamine synthetase 1 and 2). To evaluate similarities, we compared our transcriptomes with those of fly (Semper cells) and vertebrate (Müller glia) eye glia as well as respective retinas. T. marmoratus SupCs were found to have distinct genetic overlap with both fly and vertebrate eye glia. These results suggest that T. marmoratus SupCs are a form of glia, and like photoreceptors, may be deeply conserved.

Recent advances and prospects for lipid-based nanoparticles as drug carriers in the treatment of human retinal diseases

The delivery of cures for retinal diseases remains problematic. There are four main challenges: passing through multiple barriers of the eye, the delivery to particular retinal cell types, the capability to carry different forms of therapeutic cargo and long-term therapeutic efficacy. Lipid-based nanoparticles (LBNPs) are potent to overcome these challenges due to their unique merits: amphiphilic nanoarchitectures to pass biological barriers, vary modifications with specific affinity to target cell types, flexible capacity for large and mixed types of cargos and slow-release formulations for long-term treatment. We have reviewed the latest research on the applications of LBNPs for treating retinal diseases and categorized them by different payloads. Furthermore, we identified technical barriers and discussed possible future development for LBNPs to expand the therapeutic potential in treating retinal diseases.

Exploring the molecular makeup of support cells in insect camera eyes

Animals generally have either compound eyes, which have evolved repeatedly in different invertebrates, or camera eyes, which have evolved many times across the animal kingdom. Both eye types include two important kinds of cells: photoreceptor cells, which can be excited by light, and non-neuronal support cells (SupCs), which provide essential support to photoreceptors. Despite many examples of convergence in eye evolution, similarities in the gross developmental plan and molecular signatures have been discovered, even between phylogenetically distant and functionally different eye types. For this reason, a shared evolutionary origin has been considered for photoreceptors. In contrast, only a handful of studies, primarily on the compound eyes of Drosophila melanogaster , have demonstrated molecular similarities in SupCs. D. melanogaster SupCs (Semper cells and primary pigment cells) are specialized eye glia that share several molecular similarities with certain vertebrate eye glia, including Müller glia. This led us to speculate whether there are conserved molecular signatures of SupCs, even in functionally different eyes such as the image-forming larval camera eyes of the sunburst diving beetle Thermonectus marmoratus . To investigate this possibility, we used an in-depth comparative whole-tissue transcriptomics approach. Specifically, we dissected the larval principal camera eyes into SupC- and retina-containing regions and generated the respective transcriptomes. Our analysis revealed several conserved features of SupCs including enrichment of genes that are important for glial function (e.g. gap junction proteins such as innexin 3), glycogen production (glycogenin), and energy metabolism (glutamine synthetase 1 and 2). To evaluate the extent of conservation, we compared our transcriptomes with those of fly (Semper cells) and vertebrate (Müller glia) eye glia as well as respective retinas. T. marmoratus SupCs were found to have distinct genetic overlap with both fly and vertebrate eye glia. These results provide molecular evidence for the deep conservation of SupCs in addition to photoreceptor cells, raising essential questions about the evolutionary origin of eye-specific glia in animals.

The Role of Nrf2/sMAF Signalling in Retina Ageing and Retinal Diseases

Age-related diseases, such as Parkinson's disease, Alzheimer's disease, cardiovascular diseases, cancers, and age-related macular disease, have become increasingly prominent as the population ages. Oxygen is essential for living organisms, but it may also cause disease when it is transformed into reactive oxygen species via biological processes in cells. Most of the production of ROS occurs in mitochondrial complexes I and III. The accumulation of ROS in cells causes oxidative stress, which plays a crucial role in human ageing and many diseases. Nuclear factor-erythroid 2-related factor 2 (Nrf2) is a key antioxidant transcription factor that plays a central role in many diseases and ageing in general. It regulates many downstream antioxidative enzymes when cells are exposed to oxidative stress. A basic-region leucine zipper (bZIP) transcription factor, MAF, specifically the small MAF subfamily (sMAFs), forms heterodimers with Nrf2, which bind with Maf-recognition elements (MAREs) in response to oxidative stress. The role of this complex in the human retina remains unclear. This review summarises the current knowledge about Nrf2 and its downstream signalling, especially its cofactor-MAF, in ageing and diseases, with a focus on the retina. Since Nrf2 is the master regulator of redox homeostasis in cells, we hypothesise that targeting Nrf2 is a promising therapeutic approach for many age-related diseases.

Natural History of Glaucoma Progression in the DBA/2J Model: Early Contribution of Müller Cell Gliosis

Glaucoma is a chronic optic neuropathy characterized by progressive degeneration of retinal ganglion cells (RGCs). Elevated intraocular pressure (IOP) and the resulting mechanical stress are classically considered the main causes of RGC death. However, RGC degeneration and ensuing vision loss often occur independent of IOP, indicating a multifactorial nature of glaucoma, with the likely contribution of glial and vascular function. The aim of the present study was to provide a comprehensive evaluation of the time course of neuro-glial-vascular changes associated with glaucoma progression. We used DBA/2J mice in the age range of 2-15 months as a spontaneous model of glaucoma with progressive IOP elevation and RGC loss typical of human open-angle glaucoma. We found that the onset of RGC degeneration at 10 months of age coincided with that of IOP elevation and vascular changes such as decreased density, increased lacunarity and decreased tight-junction protein zonula occludens (ZO)-1, while hypoxia-inducible factor (HIF)-1α and vascular endothelial growth factor (VEGF) were already significantly upregulated at 6 months of age together with the onset of Müller cell gliosis. Astrocytes, however, underwent significant gliosis at 10 months. These results indicate that Müller cell activation occurs well before IOP elevation, with probable inflammatory consequences, and represents an early event in the glaucomatous process. Early upregulation of HIF-1α and VEGF is likely to contribute to blood retinal barrier failure, facilitating RGC loss. The different time courses of neuro-glial-vascular changes during glaucoma progression provide further insight into the nature of the disease and suggest potential targets for the development of efficient therapeutic intervention aside from IOP lowering.

Retinal TRP channels: Cell-type-specific regulators of retinal homeostasis and multimodal integration

Transient receptor potential (TRP) channels are a widely expressed family of 28 evolutionarily conserved cationic ion channels that operate as primary detectors of chemical and physical stimuli and secondary effectors of metabotropic and ionotropic receptors. In vertebrates, the channels are grouped into six related families: TRPC, TRPV, TRPM, TRPA, TRPML, and TRPP. As sensory transducers, TRP channels are ubiquitously expressed across the body and the CNS, mediating critical functions in mechanosensation, nociception, chemosensing, thermosensing, and phototransduction. This article surveys current knowledge about the expression and function of the TRP family in vertebrate retinas, which, while dedicated to transduction and transmission of visual information, are highly susceptible to non-visual stimuli. Every retinal cell expresses multiple TRP subunits, with recent evidence establishing their critical roles in paradigmatic aspects of vertebrate vision that include TRPM1-dependent transduction of ON bipolar signaling, TRPC6/7-mediated ganglion cell phototransduction, TRP/TRPL phototransduction in Drosophila and TRPV4-dependent osmoregulation, mechanotransduction, and regulation of inner and outer blood-retina barriers. TRP channels tune light-dependent and independent functions of retinal circuits by modulating the intracellular concentration of the 2nd messenger calcium, with emerging evidence implicating specific subunits in the pathogenesis of debilitating diseases such as glaucoma, ocular trauma, diabetic retinopathy, and ischemia. Elucidation of TRP channel involvement in retinal biology will yield rewards in terms of fundamental understanding of vertebrate vision and therapeutic targeting to treat diseases caused by channel dysfunction or over-activation.

Single-cell transcriptome reveals diversity of Müller cells with different metabolic-mitochondrial signatures in normal and degenerated macula

Müller cell is the most abundant glial cell in mammalian retina, supporting the functions of photoreceptors and other retinal neurons via maintaining environmental homeostasis. In response to injury and/or neuronal degeneration, Müller cells undergo morphological and functional alternations, known as reactive gliosis documented in multiple retinal diseases, including age-related macular degeneration (AMD), retinitis pigmentosa, diabetic retinopathy, and traumatic retinal detachment. But the functional consequences of Müller glia cell reactivation or even the regulatory networks of the retinal gliosis are still controversial. In this study, we reveal different subpopulations of Müller cells with distinct metabolic-mitochondrial signatures by integrating single cell transcriptomic data from Early AMD patients and healthy donors. Our results show that a portion of Müller cells exhibits low mitochondrial DNA (mtDNA) expressions, reduced protein synthesis, impaired homeostatic regulation, decreased proliferative ability but enhanced proangiogenic function. Interestingly, the major alternation of Müller cells in Early AMD retina is the change of subpopulation abundance, rather than generation of new subcluster. Transcription factor enrichment analysis further highlights the key regulators of metabolic-mitochondrial states of Müller glias in Early AMD patients especially. Our study demonstrates new characteristics of retinal gliosis associated with Early AMD and suggests the possibility to prevent degeneration by intervening mitochondrial functions of Müller cells.

Cell Sources for Retinal Regeneration: Implication for Data Translation in Biomedicine of the Eye

The main degenerative diseases of the retina include macular degeneration, proliferative vitreoretinopathy, retinitis pigmentosa, and glaucoma. Novel approaches for treating retinal diseases are based on cell replacement therapy using a variety of exogenous stem cells. An alternative and complementary approach is the potential use of retinal regeneration cell sources (RRCSs) containing retinal pigment epithelium, ciliary body, Müller glia, and retinal ciliary region. RRCSs in lower vertebrates in vivo and in mammals mostly in vitro are able to proliferate and exhibit gene expression and epigenetic characteristics typical for neural/retinal cell progenitors. Here, we review research on the factors controlling the RRCSs' properties, such as the cell microenvironment, growth factors, cytokines, hormones, etc., that determine the regenerative responses and alterations underlying the RRCS-associated pathologies. We also discuss how the current data on molecular features and regulatory mechanisms of RRCSs could be translated in retinal biomedicine with a special focus on (1) attempts to obtain retinal neurons de novo both in vivo and in vitro to replace damaged retinal cells; and (2) investigations of the key molecular networks stimulating regenerative responses and preventing RRCS-related pathologies.

Neuroinflammation in retinitis pigmentosa: Therapies targeting the innate immune system

Retinitis pigmentosa (RP) is an important cause of irreversible blindness worldwide and lacks effective treatment strategies. Although mutations are the primary cause of RP, research over the past decades has shown that neuroinflammation is an important cause of RP progression. Due to the abnormal activation of immunity, continuous sterile inflammation results in neuron loss and structural destruction. Therapies targeting inflammation have shown their potential to attenuate photoreceptor degeneration in preclinical models. Regardless of variations in genetic background, inflammatory modulation is emerging as an important role in the treatment of RP. We summarize the evidence for the role of inflammation in RP and mention therapeutic strategies where available, focusing on the modulation of innate immune signals, including TNFα signaling, TLR signaling, NLRP3 inflammasome activation, chemokine signaling and JAK/STAT signaling. In addition, we describe epigenetic regulation, the gut microbiome and herbal agents as prospective treatment strategies for RP in recent advances.

Azithromycin Protects Retinal Glia Against Oxidative Stress-Induced Morphological Changes, Inflammation, and Cell Death

The reactivity of retinal glia in response to oxidative stress has a significant effect on retinal pathobiology. The reactive glia change their morphology and secret cytokines and neurotoxic factors in response to oxidative stress associated with retinal neurovascular degeneration. Therefore, pharmacological intervention to protect glial health against oxidative stress is crucial for maintaining homeostasis and the normal function of the retina. In this study, we explored the effect of azithromycin, a macrolide antibiotic with antioxidant, immunomodulatory, anti-inflammatory, and neuroprotective properties against oxidative stress-induced morphological changes, inflammation, and cell death in retinal microglia and Müller glia. Oxidative stress was induced by H₂O₂, and the intracellular oxidative stress was measured by DCFDA and DHE staining. The change in morphological characteristics such as the surface area, perimeter, and circularity was calculated using ImageJ software. Inflammation was measured by enzyme-linked immunosorbent assays for TNF-α, IL-1β, and IL-6. Reactive gliosis was characterized by anti-GFAP immunostaining. Cell death was measured by MTT assay, acridine orange/propidium iodide, and trypan blue staining. Pretreatment of azithromycin inhibits H₂O₂-induced oxidative stress in microglial (BV-2) and Müller glial (MIO-M1) cells. We observed that azithromycin inhibits oxidative stress-induced morphological changes, including the cell surface area, circularity, and perimeter in BV-2 and MIO-M1 cells. It also inhibits inflammation and cell death in both the glial cells. Azithromycin could be used as a pharmacological intervention on maintaining retinal glial health during oxidative stress.

Cellular and molecular alterations in neurons and glial cells in inherited retinal degeneration

Multiple gene mutations have been associated with inherited retinal dystrophies (IRDs). Despite the spectrum of phenotypes caused by the distinct mutations, IRDs display common physiopathology features. Cell death is accompanied by inflammation and oxidative stress. The vertebrate retina has several attributes that make this tissue vulnerable to oxidative and nitrosative imbalance. The high energy demands and active metabolism in retinal cells, as well as their continuous exposure to high oxygen levels and light-induced stress, reveal the importance of tightly regulated homeostatic processes to maintain retinal function, which are compromised in pathological conditions. In addition, the subsequent microglial activation and gliosis, which triggers the secretion of pro-inflammatory cytokines, chemokines, trophic factors, and other molecules, further worsen the degenerative process. As the disease evolves, retinal cells change their morphology and function. In disease stages where photoreceptors are lost, the remaining neurons of the retina to preserve their function seek out for new synaptic partners, which leads to a cascade of morphological alterations in retinal cells that results in a complete remodeling of the tissue. In this review, we describe important molecular and morphological changes in retinal cells that occur in response to oxidative stress and the inflammatory processes underlying IRDs.

Photodisruption of the Inner Limiting Membrane: Exploring ICG Loaded Nanoparticles as Photosensitizers

The inner limiting membrane (ILM) represents a major bottleneck hampering efficient drug delivery to the retina after intravitreal injection. To overcome this barrier, we intend to perforate the ILM by use of a light-based approach which relies on the creation of vapor nanobubbles (VNBs) when irradiating photosensitizers with high intensity laser pulses. Upon collapse of these VNBs, mechanical effects can disrupt biological structures. As a photosensitizer, we explore indocyanine green (ICG) loaded nanoparticles (NPs) specifically designed for our application. In light of this, ICG liposomes and PLGA ICG NPs were characterized in terms of physicochemical properties, ICG incorporation and VNB formation. ICG liposomes were found to encapsulate significantly higher amounts of ICG compared to PLGA ICG NPs which is reflected in their VNB creating capacity. Since only ICG liposomes were able to induce VNB generation, this class of NPs was further investigated on retinal explants. Here, application of ICG liposomes followed by laser treatment resulted in subtle disruption effects at the ILM where zones of fully ablated ILM were alternated by intact regions. As the interaction between the ICG liposomes and ILM might be insufficient, active targeting strategies or other NP designs might improve the concept to a further extent.

Contribution of Müller Cells in the Diabetic Retinopathy Development: Focus on Oxidative Stress and Inflammation

Diabetic retinopathy is a neurovascular complication of diabetes and the main cause of vision loss in adults. Glial cells have a key role in maintenance of central nervous system homeostasis. In the retina, the predominant element is the Müller cell, a specialized cell with radial morphology that spans all retinal layers and influences the function of the entire retinal circuitry. Müller cells provide metabolic support, regulation of extracellular composition, synaptic activity control, structural organization of the blood-retina barrier, antioxidant activity, and trophic support, among other roles. Therefore, impairments of Müller actions lead to retinal malfunctions. Accordingly, increasing evidence indicates that Müller cells are affected in diabetic retinopathy and may contribute to the severity of the disease. Here, we will survey recently described alterations in Müller cell functions and cellular events that contribute to diabetic retinopathy, especially related to oxidative stress and inflammation. This review sheds light on Müller cells as potential therapeutic targets of this disease.

LncRNA MALAT1 aggravates the retinal angiogenesis via miR-320a/HIF-1α axis in diabetic retinopathy

Diabetic retinopathy (DR) is one of the most serious microvascular complications of diabetes and an important cause of blindness in adults. In previous study, we found that miR-320a alleviated the damage of muller cells in DR. In this study, we mainly explored the mechanism of lncRNA MALAT1 on retinal angiogenesis in DR by regulating miR-320a/HIF-1α. The expression of MALAT1 and miR-320a was detected by RT-qPCR, and the expression of HIF-1α was detected by Western blot. The superoxide anion level, invasion, angiogenesis, and vascular permeability of mouse retinal microvascular endothelial cells (MRMECs) co-cultured with muller cells were evaluated by dihydroethidium, transwell, angiogenesis and immunofluorescence assay. In order to analyze the targeting relationship between miR-320a and MALAT1 or HIF-1α, we performed dual luciferase reporter gene, fluorescence in situ hybridization (FISH), RNA immunoprecipitation (RIP) and RNA pulldown experiments. The results should that MALAT1 and HIF-1α were highly expressed and miR-320a was low expressed in high glucose (HG)-induced muller cells, and MALAT1 could competitively bind with HIF-1α. Knocking down miR-320a inhibited MRMECs invasion angiogenesis, and vascular permeability by targeting miR-320a. Overexpression of miR-320a down regulated HIF-1α and inhibited the invasion, angiogenesis, and vascular permeability of MRMECs. In conclusion, MALAT1 inhibits HIF-1α expression and MRMECs angiogenesis in DR through spongy miR-320a.

SIRT4 Is Highly Expressed in Retinal Müller Glial Cells

Sirtuin 4 (SIRT4) is one of seven mammalian sirtuins that possesses ADP-ribosyltransferase, lipoamidase and deacylase activities and plays indispensable role in metabolic regulation. However, the role of SIRT4 in the retina is not clearly understood. The purpose of this study was to explore the location and function of SIRT4 in the retina. Therefore, immunofluorescence was used to analyze the localization of SIRT4 in rat, mouse and human retinas. Western blotting was used to assess SIRT4 and glutamine synthetase (GS) protein expression at different developmental stages in C57BL/6 mice retinas. We further analyzed the retinal structure, electrophysiological function and the expression of GS protein in SIRT4-deficient mice. Excitotoxicity was caused by intravitreal injection of glutamate (50 nmol) in mice with long-term intraperitoneal injection of resveratrol (20 mg/Kg), and then retinas were subjected to Western blotting and paraffin section staining to analyze the effect of SIRT4 on excitotoxicity. We show that SIRT4 co-locates with Müller glial cell markers (GS and vimentin). The protein expression pattern of SIRT4 was similar to that of GS, and both increased with development. There were no significant retinal structure or electrophysiological function changes in 2-month SIRT4-deficient mice, while the expression of GS protein was decreased. Moreover, long-term administration of resveratrol can upregulate the expression of SIRT4 and GS while reducing the retinal injury caused by excessive glutamate. These results suggest that SIRT4 is highly expressed in retinal Müller glial cells and is relevant to the expression of GS. SIRT4 does not appear to be essential in retinal development, but resveratrol, as an activator of SIRT4, can upregulate GS protein expression and protect the retina from excitotoxicity.

ARPE-19 conditioned medium promotes neural differentiation of adipose-derived mesenchymal stem cells

BACKGROUND

Adipose-derived stem cells (ASCs) have been increasingly explored for cell-based medicine because of their numerous advantages in terms of easy availability, high proliferation rate, multipotent differentiation ability and low immunogenicity. In this respect, they have been widely investigated in the last two decades to develop therapeutic strategies for a variety of human pathologies including eye disease. In ocular diseases involving the retina, various cell types may be affected, such as Müller cells, astrocytes, photoreceptors and retinal pigment epithelium (RPE), which plays a fundamental role in the homeostasis of retinal tissue, by secreting a variety of growth factors that support retinal cells.

AIM

To test ASC neural differentiation using conditioned medium (CM) from an RPE cell line (ARPE-19).

METHODS

ASCs were isolated from adipose tissue, harvested from the subcutaneous region of healthy donors undergoing liposuction procedures. Four ASC culture conditions were investigated: ASCs cultured in basal Dulbecco's Modified Eagle Medium (DMEM); ASCs cultured in serum-free DMEM; ASCs cultured in serum-free DMEM/F12; and ASCs cultured in a CM from ARPE-19, a spontaneously arising cell line with a normal karyotype derived from a human RPE. Cell proliferation rate and viability were assessed by crystal violet and MTT assays at 1, 4 and 8 d of culture. At the same time points, ASC neural differentiation was evaluated by immunocytochemistry and western blot analysis for typical neuronal and glial markers: Nestin, neuronal specific enolase (NSE), protein gene product (PGP) 9.5, and glial fibrillary acidic protein (GFAP).

RESULTS

Depending on the culture medium, ASC proliferation rate and viability showed some significant differences. Overall, less dense populations were observed in serum-free cultures, except for ASCs cultured in ARPE-19 serum-free CM. Moreover, a different cell morphology was seen in these cultures after 8 d of treatment, with more elongated cells, often showing cytoplasmic ramifications. Immunofluorescence results and western blot analysis were indicative of ASC neural differentiation. In fact, basal levels of neural markers detected under control conditions significantly increased when cells were cultured in ARPE-19 CM. Specifically, neural marker overexpression was more marked at 8 d. The most evident increase was observed for NSE and GFAP, a modest increase was observed for nestin, and less relevant changes were observed for PGP9.5.

CONCLUSION

The presence of growth factors produced by ARPE-19 cells in tissue culture induces ASCs to express neural differentiation markers typical of the neuronal and glial cells of the retina.

Dietary Supplement Enriched in Antioxidants and Omega-3 Promotes Glutamine Synthesis in Müller Cells: A Key Process against Oxidative Stress in Retina

To prevent ocular pathologies, new generation of dietary supplements have been commercially available. They consist of nutritional supplement mixing components known to provide antioxidative properties, such as unsaturated fatty acid, resveratrol or flavonoids. However, to date, only one preclinical study has evaluated the impact of a mixture mainly composed of those components (Nutrof Total^®) on the retina and demonstrated that in vivo supplementation prevents the retina from structural and functional injuries induced by light. Considering the crucial role played by the glial Müller cells in the retina, particularly to regulate the glutamate cycle to prevent damage in oxidative stress conditions, we questioned the impact of this ocular supplement on the glutamate metabolic cycle. To this end, various molecular aspects associated with the glutamate/glutamine metabolism cycle in Müller cells were investigated on primary Müller cells cultures incubated, or not, with the commercially mix supplement before being subjected, or not, to oxidative conditions. Our results demonstrated that in vitro supplementation provides guidance of the glutamate/glutamine cycle in favor of glutamine synthesis. These results suggest that glutamine synthesis is a crucial cellular process of retinal protection against oxidative damages and could be a key step in the previous in vivo beneficial results provided by the dietary supplementation.

Metformin Protects against NMDA-Induced Retinal Injury through the MEK/ERK Signaling Pathway in Rats

Metformin, an anti-hyperglycemic drug of the biguanide class, exerts positive effects in several non-diabetes-related diseases. In this study, we aimed to examine the protective effects of metformin against N-methyl-D-aspartic acid (NMDA)-induced excitotoxic retinal damage in rats and determine the mechanisms of its protective effects. Male Sprague–Dawley rats (7 to 9 weeks old) were used in this study. Following intravitreal injection of NMDA (200 nmol/eye), the number of neuronal cells in the ganglion cell layer and parvalbumin-positive amacrine cells decreased, whereas the number of CD45-positive leukocytes and Iba1-positive microglia increased. Metformin attenuated these NMDA-induced responses. The neuroprotective effect of metformin was abolished by compound C, an inhibitor of AMP-activated protein kinase (AMPK). The AMPK activator, AICAR, exerted a neuroprotective effect in NMDA-induced retinal injury. The MEK1/2 inhibitor, U0126, reduced the neuroprotective effect of metformin. These results suggest that metformin protects against NMDA-induced retinal neurotoxicity through activation of the AMPK and MEK/extracellular signal-regulated kinase (ERK) signaling pathways. This neuroprotective effect could be partially attributable to the inhibitory effects on inflammatory responses.

VEGF-B Is an Autocrine Gliotrophic Factor for Müller Cells under Pathologic Conditions

Purpose

Müller glia are important in retinal health and disease and are a major source of retinal VEGF-A. Of the different VEGF family members, the role of VEGF-A in retinal health and disease has been studied extensively. The potential contribution of other VEGF family members to retinal pathophysiology, however, remains poorly defined. This study aimed to understand the role of VEGF-B in Müller cell pathophysiology.

Methods

The expression of different VEGFs and their receptors in human MIO-M1 and mouse QMMuC-1 Müller cell lines and primary murine Müller cells was examined by RT-PCR, ELISA, and Western blot. The effect of recombinant VEGF-B or VEGF-B neutralization on Müller cell viability and survival under normal, hypoxic, and oxidative (4-hydroxynonenal [4-HNE]) conditions was evaluated by Alamar Blue, Yo-Pro uptake, and immunocytochemistry. The expression of glial fibrillary acidic protein, aquaporin-4, inward rectifying K⁺ channel subtype 4.1, glutamine synthetase, and transient receptor potential vanilloid 4 under different treatment conditions was examined by RT-PCR, immunocytochemistry, and Western blot. Transient receptor potential vanilloid 4 channel activity was assessed using a Fura-2–based calcium assay.

Results

VEGF-B was expressed in Müller cells at the highest levels compared with other members of the VEGF family. VEGF-B neutralization did not affect Müller cell viability or functionality under normal conditions, but enhanced hypoxia– or 4-HNE–induced Müller cell death and decreased inward rectifying K⁺ channel subtype 4.1 and aquaporin-4 expression. Recombinant VEGF-B restored Müller cell glutamine synthetase expression under hypoxic conditions and protected Müller cells from 4-HNE–induced damage by normalizing transient receptor potential vanilloid 4 channel expression and activity.

Conclusions

Autocrine production of VEGF-B protects Müller cells under pathologic conditions.

Chemical induction of neurogenic properties in mammalian Müller glia

Müller glia (MG), cells that maintain homeostasis in the retina, are dormant stem cells that can regenerate neurons upon injury. However, the regenerative property of MG, which is reproducibly displayed in the lower vertebrates, is not readily observed in the mammals even upon forced expression of regulatory genes or exposure to growth factors. Here, we demonstrate a reproducible unmasking of the neurogenic properties of enriched rodent MG by serial exposure to different combinations of small molecules. The enriched MG, in response to changing culture conditions, silenced glia-specific genes and acquired transcriptional signature of neurons, accompanied by upregulation of genes known to regulate neuronal potential of MG. The MG-derived neurons expressed immunoreactivities corresponding to neuronal proteins and displayed electrophysiological features of immature neurons. Our study presents a proof of principle of pharmacological activation of neurogenic properties of mammalian MG, which may be utilized for therapeutic regeneration.

Cell Surface Profiling of Retinal Müller Glial Cells Reveals Association to Immune Pathways after LPS Stimulation

Retinal Müller glial cells (RMG) are involved in virtually every retinal disease; however, the role of these glial cells in neuroinflammation is still poorly understood. Since cell surface proteins play a decisive role in immune system signaling pathways, this study aimed at characterizing the changes of the cell surface proteome of RMG after incubation with prototype immune system stimulant lipopolysaccharide (LPS). While mass spectrometric analysis of the human Müller glia cell line MIO-M1 revealed 507 cell surface proteins in total, with 18 proteins significantly more abundant after stimulation (ratio ≥ 2), the surfaceome of primary RMG comprised 1425 proteins, among them 79 proteins with significantly higher abundance in the stimulated state. Pathway analysis revealed notable association with immune system pathways such as “antigen presentation”, “immunoregulatory interactions between a lymphoid and a non-lymphoid cell” and “cell migration”. We could demonstrate a higher abundance of proteins that are usually ascribed to antigen-presenting cells (APCs) and function to interact with T-cells, suggesting that activated RMG might act as atypical APCs in the course of ocular neuroinflammation. Our data provide a detailed description of the unstimulated and stimulated RMG surfaceome and offer fundamental insights regarding the capacity of RMG to actively participate in neuroinflammation in the retina.

Regulation of Endothelium-Reticulum-Stress-Mediated Apoptotic Cell Death by a Polymethoxylated Flavone, Nobiletin, Through the Inhibition of Nuclear Translocation of Glyceraldehyde 3-Phosphate Dehydrogenase in Retinal Müller Cells

In the early stages of diabetic retinopathy (DR), subtle biochemical and functional alterations occur in Müller cells, which are one of the components of the blood-retinal barrier (BRB). Müller cells are the principal glia of the retina and have shown a strong involvement in the maintenance of homeostasis and the development of retinal tissue. Their functional abnormalities and eventual loss have been correlated with a decrease in the tight junctions between endothelial cells and a consequent breakdown of the BRB, leading to the development of DR. We demonstrated that the endothelium reticulum (ER) triggers Müller cell death and that nuclear accumulation of glyceraldehyde 3-phosphate dehydrogenase is closely associated with ER-induced Müller cell death. In addition, induction of ER stress in Müller cells increased vascular endothelial growth factor expression but decreased pigment-epithelium-derived factor (PEDF) expression in Müller cells. We found that nobiletin, a polymethoxylated flavone from citrus explants, exerts protective action against ER-stress-induced Müller cell death. In addition, nobiletin was found to augment PEDF expression in Müller cells, which may lead to the protection of BRB integrity. These results suggest that nobiletin can be an attractive candidate for the protection of the BRB from breakdown in DR.

The road to restore vision with photoreceptor regeneration

Neuroretinal diseases are the predominant cause of irreversible blindness worldwide, mainly due to photoreceptor loss. Currently, there are no radical treatments to fully reverse the degeneration or even stop the disease progression. Thus, it is urgent to develop new biological therapeutics for these diseases on the clinical side. Stem cell-based treatments have become a promising therapeutic for neuroretinal diseases through the replacement of damaged cells with photoreceptors and some allied cells. To date, considerable efforts have been made to regenerate the diseased retina based on stem cell technology. In this review, we overview the current status of stem cell-based treatments for photoreceptor regeneration, including the major cell sources derived from different stem cells in pre-clinical or clinical trial stages. Additionally, we discuss herein the major challenges ahead for and potential new strategy toward photoreceptor regeneration.

YAP is essential for TGF-β-induced retinal fibrosis in diabetic rats via promoting the fibrogenic activity of Müller cells

The purpose of this study was to investigate whether Yes-associated protein (YAP) activation and proliferation of retinal Müller cells play a role in the development of TGF-β-induced retinal fibrosis. We studied the effects of YAP activation on retinal fibrosis in diabetic rats and human retinal Müller cells (hMCs) in vitro. The retinal expression of YAP and fibrogenic molecules in rats was detected using Western blotting and immunohistochemistry. After treatment with transforming growth factor-β1 (TGF-β1), the levels of fibrogenic molecules, and the activation of YAP and PI3K/Akt signalling pathway in hMCs were detected with Western blotting. The effect of YAP on retinal fibrotic changes was evaluated using YAP knockdown experiments and YAP inhibitors. Results showed that YAP expression was increased in the retina of diabetic rats along with increased retinal fibrosis. In cultured hMCs, YAP inhibition suppressed TGF-β1-stimulated hMC differentiation to myofibroblasts and extracellular matrix (ECM) production, while YAP activation promoted hMC differentiation and ECM production independent of TGF-β1. Furthermore, hMCs cultured on a gel with greater stiffness differentiated into myofibroblasts in a YAP-dependent manner. In diabetic rats, treatment with the YAP inhibitor verteporfin suppressed retinal fibrogenesis. In addition, the TGF-β1-induced PI3K/Akt signalling pathway mediated YAP activation as well as expression of fibrogenic molecules. The interaction between ECM stiffness and YAP forms a feed-forward process leading to retinal fibrosis. Our work highlights YAP as an essential regulator of pro-fibrotic responses in TGF-β-induced retinal fibrosis.

Potential Endogenous Cell Sources for Retinal Regeneration in Vertebrates and Humans: Progenitor Traits and Specialization

Retinal diseases often cause the loss of photoreceptor cells and, consequently, impairment of vision. To date, several cell populations are known as potential endogenous retinal regeneration cell sources (RRCSs): the eye ciliary zone, the retinal pigment epithelium, the iris, and Müller glia. Factors that can activate the regenerative responses of RRCSs are currently under investigation. The present review considers accumulated data on the relationship between the progenitor properties of RRCSs and the features determining their differentiation. Specialized RRCSs (all except the ciliary zone in low vertebrates), despite their differences, appear to be partially "prepared" to exhibit their plasticity and be reprogrammed into retinal neurons due to the specific gene expression and epigenetic landscape. The "developmental" characteristics of RRCS gene expression are predefined by the pathway by which these cell populations form during eye morphogenesis; the epigenetic features responsible for chromatin organization in RRCSs are under intracellular regulation. Such genetic and epigenetic readiness is manifested in vivo in lower vertebrates and in vitro in higher ones under conditions permissive for cell phenotype transformation. Current studies on gene expression in RRCSs and changes in their epigenetic landscape help find experimental approaches to replacing dead cells through recruiting cells from endogenous resources in vertebrates and humans.

Update on Müller glia regenerative potential for retinal repair

Retinal regeneration efficiency from Müller glia varies tremendously among vertebrate species, being extremely limited in mammals. Efforts towards the identification of molecular mechanisms underlying Müller cell proliferative and neurogenic potential should help finding strategies to awake them and ensure regeneration in mammals. We provide here an update on the most recent and original progresses made in the field. These include remarkable discoveries regarding (i) unprecedented cross-species comparison of Müller cell transcriptome using single-cell technologies, (ii) the identification of new strategies to promote both the proliferative and the neurogenic potential of mammalian Müller cells, (iii) the role of the epigenome in regulating Müller glia plasticity, (iv) miRNA-based regulatory mechanisms of Müller cell response to injury, and (v) the influence of inflammatory signals on the regenerative process.

Oxidative Stress Induces a VEGF Autocrine Loop in the Retina: Relevance for Diabetic Retinopathy

BACKGROUND

Oxidative stress (OS) plays a central role in diabetic retinopathy (DR), triggering expression and release of vascular endothelial growth factor (VEGF), the increase of which leads to deleterious vascular changes. We tested the hypothesis that OS-stimulated VEGF induces its own expression with an autocrine mechanism.

METHODS

MIO-M1 cells and ex vivo mouse retinal explants were treated with OS, with exogenous VEGF or with conditioned media (CM) from OS-stressed cultures.

RESULTS

Both in MIO-M1 cells and in retinal explants, OS or exogenous VEGF induced a significant increase of VEGF mRNA, which was abolished by VEGF receptor 2 (VEGFR-2) inhibition. OS also caused VEGF release. In MIO-M1 cells, CM induced VEGF expression, which was abolished by a VEGFR-2 inhibitor. Moreover, the OS-induced increase of VEGF mRNA was abolished by a nuclear factor erythroid 2-related factor 2 (Nrf2) blocker, while the effect of exo-VEGF resulted Nrf2-independent. Finally, both the exo-VEGF- and the OS-induced increase of VEGF expression were blocked by a hypoxia-inducible factor-1 inhibitor.

CONCLUSIONS

These results are consistent with the existence of a retinal VEGF autocrine loop triggered by OS. This mechanism may significantly contribute to the maintenance of elevated VEGF levels and therefore it may be of central importance for the onset and development of DR.

Influence of pathogenic stimuli on Müller cell transfection by lipoplexes

Neuroprotection is a mutation-independent therapeutic strategy that seeks to enhance the survival of neuronal cell types through delivery of neuroprotective factors. The Müller cell, a retinal glial cell type appreciated for its unique morphology and neuroprotective functions, could be regarded as an ideal target for this strategy by functioning as a secretion platform within the retina following uptake of a transgene of our choice. In this in vitro study we aimed to investigate the capability of Müller cells to take up a standard liposomal vector (i.e. Lipofectamine 2000) and process its pDNA or mRNA cargo into the reporter GFP protein. By doing so, we found that mRNA outperformed pDNA in Müller cell transfection efficiency. Since neuroprotection is explored as a therapy for diabetic retinopathy and glaucoma, we furthermore examined the Müller cell's lipoplex-induced transfection efficiency and cytotoxicity in stressful conditions linked to these diseases - i.e. hypoxia, hyperglycemia and oxidative stress. Interestingly, Müller cells were able of maintaining high GFP expression regardless of these noxious stimuli. In terms of lipoplex-induced toxicity, hyperglycemia seemed to have a protective effect while hypoxia and oxidative stress led to a slightly higher toxicity. In conclusion, our study indicates that mRNA-lipoplexes have potential in transfecting Müller cells in healthy as well as diseased conditions.

The CD40-ATP-P2X 7 Receptor Pathway: Cell to Cell Cross-Talk to Promote Inflammation and Programmed Cell Death of Endothelial Cells

Extracellular adenosine 5′-triphosphate (ATP) functions not only as a neurotransmitter but is also released by non-excitable cells and mediates cell–cell communication involving glia. In pathological conditions, extracellular ATP released by astrocytes may act as a “danger” signal that activates microglia and promotes neuroinflammation. This review summarizes in vitro and in vivo studies that identified CD40 as a novel trigger of ATP release and purinergic-induced inflammation. The use of transgenic mice with expression of CD40 restricted to retinal Müller glia and a model of diabetic retinopathy (a disease where the CD40 pathway is activated) established that CD40 induces release of ATP in Müller glia and triggers in microglia/macrophages purinergic receptor-dependent inflammatory responses that drive the development of retinopathy. The CD40-ATP-P2X₇ pathway not only amplifies inflammation but also induces death of retinal endothelial cells, an event key to the development of capillary degeneration and retinal ischemia. Taken together, CD40 expressed in non-hematopoietic cells is sufficient to mediate inflammation and tissue pathology as well as cause death of retinal endothelial cells. This process likely contributes to development of degenerate capillaries, a hallmark of diabetic and ischemic retinopathies. Blockade of signaling pathways downstream of CD40 operative in non-hematopoietic cells may offer a novel means of treating diabetic and ischemic retinopathies.

Clarin-1 expression in adult mouse and human retina highlights a role of Müller glia in Usher syndrome

Usher syndrome type 3 (USH3) is an autosomal recessively inherited disorder caused by mutations in the gene clarin‐1 (CLRN1), leading to combined progressive hearing loss and retinal degeneration. The cellular distribution of CLRN1 in the retina remains uncertain, either because its expression levels are low or because its epitopes are masked. Indeed, in the adult mouse retina, Clrn1 mRNA is developmentally downregulated, detectable only by RT‐PCR. In this study we used the highly sensitive RNAscope in situ hybridization assay and single‐cell RNA‐sequencing techniques to investigate the distribution of Clrn1 and CLRN1 in mouse and human retina, respectively. We found that Clrn1 transcripts in mouse tissue are localized to the inner retina during postnatal development and in adult stages. The pattern of Clrn1 mRNA cellular expression is similar in both mouse and human adult retina, with CLRN1 transcripts being localized in Müller glia, and not photoreceptors. We generated a novel knock‐in mouse with a hemagglutinin (HA) epitope‐tagged CLRN1 and showed that CLRN1 is expressed continuously at the protein level in the retina. Following enzymatic deglycosylation and immunoblotting analysis, we detected a single CLRN1‐specific protein band in homogenates of mouse and human retina, consistent in size with the main CLRN1 isoform. Taken together, our results implicate Müller glia in USH3 pathology, placing this cell type to the center of future mechanistic and therapeutic studies to prevent vision loss in this disease. © 2019 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

Activated α2-Macroglobulin Regulates LRP1 Levels at the Plasma Membrane through the Activation of a Rab10-dependent Exocytic Pathway in Retinal Müller Glial Cells

Activated α₂-macroglobulin (α₂M*) and its receptor, low-density lipoprotein receptor-related protein 1 (LRP1), have been linked to proliferative retinal diseases. In Müller glial cells (MGCs), the α₂M*/LRP1 interaction induces cell signaling, cell migration, and extracellular matrix remodeling, processes closely associated with proliferative disorders. However, the mechanism whereby α₂M* and LRP1 participate in the aforementioned pathologies remains incompletely elucidated. Here, we investigate whether α₂M* regulates both the intracellular distribution and sorting of LRP1 to the plasma membrane (PM) and how this regulation is involved in the cell migration of MGCs. Using a human Müller glial-derived cell line, MIO-M1, we demonstrate that the α₂M*/LRP1 complex is internalized and rapidly reaches early endosomes. Afterward, α₂M* is routed to degradative compartments, while LRP1 is accumulated at the PM through a Rab10-dependent exocytic pathway regulated by PI3K/Akt. Interestingly, Rab10 knockdown reduces both LRP1 accumulation at the PM and cell migration of MIO-M1 cells induced by α₂M*. Given the importance of MGCs in the maintenance of retinal homeostasis, unravelling this molecular mechanism can potentially provide new therapeutic targets for the treatment of proliferative retinopathies.

A rAAV2/6 Mutant with Enhanced Targeting for Mouse Retinal Müller Cells

Purpose: Adeno-associated virus vector (AAV) is the most accepted gene delivery vector for retinal gene therapy. Müller cells play an important role in maintaining homeostasis and neuronal structural integrity, stability and it has been found to be involved in many retinopathies. The aim of this study is to identify a rAAV2/6 mutant which has increased tropism for Müller cell of the mouse retina.Materials and Methods: Using amino acid mutagenesis, we created a rAAV2/6 capsid mutant, rAAV2/6-S663L. In vivo imaging and retinal flat mount were employed to analyze the gene expression of rAAV2/6-S663L and wt rAAV2/6 in mouse retinal tissue. Retinal tissue cryosection, immunohistochemistry (IHC), Müller cell-specific promoter-controlled gene expression, and double AAV fluorescent protein co-expression were performed to determine the targeting of rAAV2/6-S663L for mouse retinal Müller cells.Results: In vivo imaging, retinal flat mount and retinal tissue cryosection results showed that rAAV2/6-S663L and wt rAAV2/6 have different specific tropisms in mouse retina and rAAV2/6-S663L is more preferentially targeting Müller cells. Müller cell-specific promoter-controlled gene expression experiments and IHC test confirmed that rAAV2/6-S663L has a higher tendency to infect Müller cells than wt rAAV2/6. Co-infection of the mouse retina with one rAAV2/6-S663L expressing EGFP under the control of GFAP promoter and the other one expressing mCherry under the control of CMV promoter revealed co-expression of the two fluorescent proteins in Müller cells.Conclusions: The results confirmed that rAAV2/6-S663L has a higher tropism for Müller cells than wt rAAV2/6. Our findings could add a new useful tool for retinal disease gene therapy.

The obstacle course to the inner retina: Hyaluronic acid-coated lipoplexes cross the vitreous but fail to overcome the inner limiting membrane

Considerable research over the last few years has revealed dysregulation of growth factors in various retinal diseases, such as glaucoma, diabetic retinopathy and photoreceptor degenerations. The use of messengerRNA (mRNA) to transiently overexpress a specific factor could compensate for this imbalance. However, a critical challenge of this approach lies in the ability to efficiently deliver mRNA molecules to the retinal target cells. In this study we found that intravitreal (IVT) injection is an attractive approach to deliver mRNA to the retina, providing two critical barriers can be overcome: the vitreous and the inner limiting membrane (ILM). We demonstrated that the vitreous is indeed a major hurdle in the delivery of the cationic mRNA-complexes to retinal cells, both in terms of vitreal mobility and cellular uptake. To improve their intravitreal mobility and avoid unwanted extracellular interactions, we evaluated the use of hyaluronic acid (HA) as an electrostatic coating strategy. This HA-coating provided the complexes with a negative surface charge, markedly enhancing their mobility in the vitreous humor, without reducing their cellular internalization and transfection efficiency. However, although this coating strategy allows the mRNA-complexes to successfully overcome the vitreal barrier, the majority of the particles accumulated at the ILM. This study therefore underscores the crucial barrier function of the ILM toward non-viral retinal gene delivery and the need to smartly design mRNA-carriers able to surmount the vitreous as well as the ILM.

miR-29c regulates neurogliogenesis in the mammalian retina through REST

In the developing central nervous system, including its simple and accessible model retina, neurogenesis is followed by gliogenesis. However, the mechanism underlying the neurogliogenic switch remains poorly understood despite the identification of several regulatory genes, associated with the lineage identity and transition. The mechanism may involve cross talks between regulatory genes, facilitated through microRNAs. Here, we posit miR-29c as one of the regulatory miRNAs that may influence neuronal versus glial differentiation. We observed that the temporal patterns of miR-29c expression corresponded with late retinal histogenesis, the stage in the developing retina when neurogliogenic decision predominantly occurs. Examination of the effects of miR-29c on neurogliogenesis by the perturbation of function approach revealed that miR-29c preferentially facilitated differentiation of late RPCs into rod photoreceptors and bipolar cells, the late-born neurons, at the expense of Müller glia, the sole glia generated by retinal progenitor cells. We further observed that miR-29c facilitated neurogenesis and inhibited gliogenesis by regulating the expression of RE-1 silencing transcription factor (REST), which encodes a transcriptional repressor of cell cycle regulators and neuronal genes. Thus, miR-29c may influence neurogliogenic decision in the developing retina by regulating the instructive out put of a molecular axis helmed by REST.